This invention relates to a fuel control system and a valve therefor. More particularly, but not exclusively, the invention relates to such a system and valve for use with a gas turbine engine for aerospace applications. The fuel supply system for a gas turbine engine generally comprises a high-pressure fuel pump and series metering valve. In order to provide consistent, accurate, metering of fuel, a spill valve is provided to maintain a substantially constant pressure drop across the metering valve. Where the output flow rate from the pump significantly exceeds the metered flow requirement, a large volume of fuel is returned by the spill valve to the pump islet. This high recirculated volume results in undesirable heat rejection into the fuel.
In accordance with the invention, a fuel control system for an engine comprises a pump operable to supply an output flow of fuel, engine fuel flow regulation means operable to regulate the pump output flow, and pump control means operable to vary an output rate of the pump according to an operational condition of the engine fuel flow regulation means.
Conveniently, the engine fuel flow regulation means comprises a metering valve, and the pump control means is operable to vary the output rate of the pump according to the operational condition of the metering valve.
A fuel control system for an engine comprising a pump operable to supply an output flow of fuel, a metering valve operable to regulate the output flow, and auxiliary flow control means operable to vary an auxiliary flow of fuel from the output flow according to an operational condition of the metering valve.
Additionally, a pressure drop and spill valve may be connected across the metering valve. This facilitates accurate and rapidly responsive control of the regulated flow, which complements the relatively slow control of the output flow provided by the pump control means.
Preferably, the pump control means comprises a servo device for varying the output rate of the pump in response to a servo flow, and a servo flow port arrangement integral with the metering valve for controlling the servo flow to the servo device. Appropriate pump control can be provided in this manner without the need for a discrete servo valve arrangement, facilitating a reduction of the volume and weight of the fuel control system.
Conveniently, the pump control means is operable such that the pump output rate is increased to a predetermined maximum when a low pressure servo flow is supplied to the servo device and decreased to a predetermined minimum when a high pressure servo flow is supplied to the servo device. This alleviates the potential problem of a depiction of the output flow to the engine by a high-pressure servo flow when the engine requires maximum available regulated flow.
The engine fuel flow regulation means may comprise a pressure drop and spill valve operable to regulate a pressure drop across a metering valve by spilling fuel, tie control means being operable to vary the output rate of the pump according to the operational condition of the pressure drop and spill valve.
The pressure drop and spill valve facilitates accurate and rapid fine tuning of the system parameters whilst the pump control means facilitates the provision of slightly slower, more approximate continuous output flow adjustments to reduce spill volume, with a view to achieving near optimum pump delivery and heat rejection across a wide range of operating conditions.
Preferably, the pump control means comprises a servo device for varying the output rate of the pump in response to a servo flow, and a servo flow port arrangement integral with the pressure drop and spill valve for controlling the servo flow to the device. Appropriate pump control can be provided in this manner without the need for a separate servo valve and/or servo position sensor arrangement, facilitating a reduction in the volume and weight of the fuel control system.
In another aspect, the invention provides a metering valve for a fuel control system or a pressure drop and spill valve for a fuel control system, the valve including an auxiliary flow port arrangement for varying an auxiliary flow from the valve to a bearing surface of a fuel pump of the system and/or a servo flow port arrangement for controlling a servo flow from the valve to a fuel pump control servo device.
According to a further aspect of the invention, there is provided a fuel control system for an engine, comprising. a pump operable to supply an output flow of fuel, a valve operable to regulate the output flow, and auxiliary flow control means operable to vary an auxiliary flow of fuel from the output flow according to an operational condition of the valve. This permits reduction of the auxiliary flow to minimise depletion of the output flow and maximise the regulated flow available to the engine.
The auxiliary flow control means may include an auxiliary flow port arrangement integral with the valve, for varying the auxiliary flow. This removes the need for a separate auxiliary flow control valve and/or sensor, facilitating a reduction in the volume and weight of the fuel control system. The term xe2x80x9cengine fuel flow regulation meansxe2x80x9d as used herein refers to any means suitable for regulating pump output flow, for example a metering valve and/or pressure drop and spill valve.